JEE Main 2022 · 28 Jun · Shift 1, Mathematics question 21

Let $R_1$ and $R_2$ be relations on the set $\{1,2,\dots,50\}$ such that

$R_1=\left\{\left(p,p^n\right):p\text{ is a prime and }n\ge0\text{ is an integer}\right\}$ and

$R_2=\left\{\left(p,p^n\right):p\text{ is a prime and }n=0\text{ or }1\right\}$.

Then, the number of elements in $R_1-R_2$ is ______.